There is accumulating evidence that the absence of C5 in mice leads to intensified lung inflammatory responses in the asthma model. Our own data suggest that generation of C5a in lung after deposition of IgG immune complexes (i. c.) results in both a pro-inflammatory cascade (e.g. production of TNFalpha, IL-1, CXC chemokines, etc.) and an anti-inflammatory cascade (e.g. production and/or activation of IL-10, IL-13, STAT3 and SOCS3). Using the IgG i. c. and lipopolysaccharide (LPS) models of acute lung injury (ALI) in mice, we will pursue preliminary information that C5a blockade in vivo attenuates injury in the former and intensifies injury in the latter and determine if measurements of the pro- and anti-inflammatory mediators correlate with ALI outcomes. Using the mouse alveolar macrophage cell line (MHS), we will use siRNA silencing technology for STAT3 and SOCS3 to assess how cytokine and chemokine responses to IgG i. c. and LPS with or without C5a are affected in the presence or absence of STAT3 or SOCS3. Using our recently developed blocking antibodies to the two C5a receptors (C5aR, C5L2), we will determine how each antibody affects binding of 125I-C5a to rodent PMN and alveolar macrophages and how these antibodies affect cytokine and chemokine responses of these cells after addition of LPS or IgG i. c. in the presence or absence of C5a. Finally, we will evaluate lung inflammatory responses of mice in which adenoviral vector technology or mRNA silencing approaches for C5aR and C5L2 have been employed. We expect that these studies will provide important information on pro-inflammatory and anti-inflammatory cascades in lung and the roles of C5aR and C5L2 in the lung inflammatory response.